The present invention concerns the MLS-type landing systems. Its object is more particularly centralized generation of the microwave signals which must be transmitted by the different stations of an MLS.
We recall that the Microwave Landing System (MLS) is a system enabling an aircraft to be guided in to land by providing it with various items of information known as "functions". These comprise angular information, such as its angle of azimuth with respect to the axis of the runway, its angle of elevation with respect to the horizontal, if necessary other information such as rear azimuth for example, and a number of data, some of them "basic" and others "auxiliary". All this information is transmitted permanently from ground level in time-sharing at the same frequency, according to characteristics standardized by the ICAO (International Civil Aviation Organization), appendix 10 paragraph III.11. The ICAO's definition of an MLS also includes a DME (Distance Measuring Equipment). All this information is decoded by every interested aircraft.
Each of the above functions is divided into two parts, which are transmitted one after the other:
A preamble, whose role is to provide the aircraft with an identification of the function which is to follow. This preamble is transmitted by a sectorial antenna, i.e. a fixed antenna transmitting to the whole of the zone, or sector, which the MLS must cover. According to the ICAO standards, the preamble is in the form of a twelve-bit word enabling biunique identification of each function. This binary word is transmitted in DPSK phase modulation (Differential Phase Shift Keying).
The function itself: if this function is a datum, it is transmitted by the sectorial antenna also in DPSK phase modulation. If it is an item of angular information it is constituted of two pulses transmitted using an electronic scanning antenna, according to the principle known as the time-reference scanning beam, which is notably described in French patent application no. 2 549 321 in the name of THOMSON-CSF.
An MLS therefore has at least as many stations as angular functions to be transmitted.
It is seen then that the various items of information required for guidance of an aircraft are transmitted by different stations in succession at the same frequency. It is clear that the order and duration of the transmissions must be strictly adhered to for guidance safety to be ensured, especially during landing, which is a particularly delicate operation. In particular, any overlapping of transmissions must be carefully avoided. To guarantee this security, many devices and solutions are known which make use of:
great redundancy of equipment;
synchronization links between the stations, preferably also redundant;
numerous surveillance loops checking the transmissions.
These different solutions tend to multiply and complicate the equipment and circuits; their major disadvantage is that they reduce the reliability of the system.